By Renee Duffy
Modern methods of remote acquisition, image data processing and modeling have presented new opportunities to research and better understand the complexities of seagrass ecology. The high spatial and spectral resolution information provided by modern airborne sensors such as satellite imagery presents opportunities to monitor subtle yet ecologically important changes in seagrass abundance.
In 2016, FWRI’s Center for Spatial Analysis received funds from FDEP’s Coastal Management Program to quantify long term changes in seagrass cover within the Indian River Lagoon (IRL) system. The demand for improved mapping and monitoring submerged resources in the IRL was driven, in part, by unprecedented losses in seagrass resulting from several algal ‘super bloom’ events starting in 2011.
The study employed a supervised classification method to map seagrass percent cover by relating spectral values in the satellite image to percent cover observations collected at fixed station transects by the St. John’s River Water Management District (SJRWMD) and the Ecological Program at NASA’s Kennedy Space Center.
Results indicated considerable declines in percent cover between 2010 and 2016. The extent of seagrass loss, however, was spatially and temporally variable throughout the lagoon system. Throughout the study area, seagrass cover was highest in 2010 and 2011 just prior to the 2011 super bloom and then declined considerably in 2012. In several areas, there were signs of recovery with increased percent cover in 2013, however, seagrass declined in 2015 and was nearly absent in most areas by 2016.
Declines in seagrass percent cover were highest in the southern portion of the Indian River Lagoon and throughout the Banana River with as much as 100% loss of the densest seagrass (75-100% cover). While complete loss of seagrass was observed by 2015 and 2016 in some areas, there was a general pattern of “thinning” in seagrass percent cover throughout the study area. This pattern is characterized by replacement of dense seagrass (>50% cover) with sparse, low density seagrass (<25% cover). The extent of this variation was not detectable from small scale in situ transect monitoring nor from temporally limited aerial photography. Results of the study emphasized the need for evaluating landscape-scale variability in seagrass percent cover using a variety of remote sensing technologies.